The present invention relates generally to a filtration system and apparatus for removing particulate matter from a stream of gas or other fluid. More specifically, but not by way of limitation, the present invention encompasses a filtration system that includes a water-cooled electrostatic pre-collector apparatus and a fabric filter element for removing particulate matter from a stream of gas or other fluid.
Fabric filtration is a common technique for separating out particulate matter in a gas stream. In an industrial setting, fabric filtration is often accomplished in a device known as a baghouse. Generally, a baghouse includes a housing that has an inlet for receiving dirty, particulate-laden gas and an outlet through which clean gas leaves the baghouse. The interior of the housing is divided by a tube sheet into a dirty gas or upstream plenum and a clean gas or downstream plenum, with the dirty gas plenum in fluid communication with the inlet and the clean gas plenum in fluid communication with the outlet. The tube sheet typically includes a number of apertures and supports a number of filter elements with each filter element covering one of the apertures.
Generally, a filter element includes a support structure and a fabric filter media. The support structure, which is also called a core, typically has a cylindrical shape and is hollow. The walls of the support structure may be similar to a screen or a cage, or may simply include a number of perforations, so that a fluid may pass through the support structure. The support structure will have at least one end that is open and that is capable of being coupled to the tube sheet at an aperture. Customarily, the structure will extend from the tube sheet into the dirty gas plenum. There are several types of fabric filter media. A “bag” filter media is flexible and/or pliable and is shaped like a bag. A cartridge filter media is typically relatively rigid and pleated. Filter media are ordinarily mounted around the exterior or outer portion of the support structure.
In operation, particulate laden or dirty gas is conducted into the baghouse, and more specifically into the dirty gas plenum, through the inlet. The gas then flows through the fabric filter media to the interior space within the filter cores. As the gas flows through the filter media, the particulate matter carried by the gas engages the exterior of the filter media and either accumulates on the filters or falls to the lower portion of the dirty gas plenum. Thereafter, the cleaned gas flows through the apertures in the tube sheet and into the clean gas plenum. The clean gas then flows out of the baghouse through the outlet.
As particulate matter accumulates or cakes on the filters, the flow rate of the gas is reduced and the pressure drop across the filters increases. To restore the desired flow rate, a reverse pressure pulse may be applied to the filters. The reverse pressure pulse expands the filter media and separates the particulate matter, which falls to the lower portion of the dirty gas plenum. While filter material technology has advanced sufficiently to allow a given filter element to be cleaned in this manner tens of thousands of times before replacement is needed, further extension of a filter's useful life is economically desirable. Extended filter life not only saves the cost of filters, it also saves the cost of filter replacement, which is often difficult, costly and requires the baghouse to be taken out of service for a period of time.
Another common technique for separating particulate matter from a gas stream is to use an electrostatic device, such as an electrostatic precipitator. In this device, particulate matter is electronically charged and then collected through the action of an electric field. A typical electrostatic device provides a discharge electrode that is maintained at a high voltage and a non-discharge electrode that is maintained at a relatively lower voltage or at ground. As the particulate-laden gas stream flows past the electrodes, the electric field present between the electrodes operates to charge a percentage of the passing particulate matter and causes them to collect on the non-discharge electrode. However, as the particulate matter coats the non-discharge electrode, an electrical resistivity increases, causing further collection of charged particulate matter more and more difficult. It has been discovered that the electrical resistivity of the particulate matter coat on the electrode is a function of temperature. More specifically, as the temperature of the particulate matter coat decreases, the electrical resistivity of the particulate matter coat decreases also, which may allow further collection of particulate matter on the non-discharge electrode.
An electrostatic precipitator may be used to pre-collect particulate matter in filtering environment. However, the usefulness of an electrostatic precipitator may be minimized in an environment where the non-discharge electrode becomes quickly coated with particulate matter, such as, for example, a baghouse filter chamber of a coal-fired power plant. This may be reversed by cooling the particulate matter coating such that the resistivity remains low. Thus, there is a need for systems for cooling the non-discharge electrode to improve the pre-collection of particulate matter by the electrostatic device.